1. Field of the Invention
The present invention relates to a sheet supply apparatus for separating stacked sheets one by one and for supplying the separated sheet, and an image forming apparatus having such a sheet supply apparatus and adapted to form an image on a supplied sheet.
2. Related Background Art
In the past, recording apparatuses such as printers, copying machines, facsimile machine and the like having a sheet supply apparatus have been so designed that an image comprised of a plurality of dots is formed on a sheet such as a paper sheet, a plastic film and the like by driving a recording head in response to image information. The recording apparatus of this kind utilizes an ink jet recording method, a wire dot recording method, a thermal recording method, an electrophotographic recording method or the like. Further, the sheet used with the recording apparatus of this type may be a plain paper sheet, a thick sheet such as a post card and an envelope, or a special sheet such as a plastic film. As means for supplying the sheet, a manual insertion sheet supply means for manually supplying the sheets one by one and an automatic sheet supply means for supplying stacked sheets one by one automatically and continuously by means of an automatic sheet supply apparatus are well-known.
The sheet supply apparatus is provided with a separation means for preventing the double-feed of sheets by separating the sheets one by one, which separation means is generally constituted by a separation pawl for holding down a front corner of the sheet. In the separating method using such a separation pawl, undulation is created in the front corner portion of the sheet by the resistance of the separation pawl, thereby separating the sheets one by one. This separating method is relatively simple and is highly reliable. In such a sheet supply apparatus, when the sheets are stacked on a stacking means, an operator depresses the stacking means downwardly to increase a distance between the separation pawl and a sheet stacking surface of the stacking means so that the stacking of the sheets is not obstructed by the separation pawl. After the sheets are stacked on the stacking means, the stacking means is returned to its original position so that the front corner of the sheet stack is held down by the separation pawl.
However, when the thick sheet such as an envelope and post card having high resiliency is supplied by using the conventional sheet supply apparatus having the above-mentioned separation pawl, since the thick sheet is caught by the separation pawl to make the separation of the sheets impossible, it is necessary to retract the separation pawl out of a sheet supplying path by a manual operation by using a lever and the like. In this condition, the thick sheets are separated from each other by difference in friction force between sheet-to-sheet and sheet-to-stacking means. Thus, when the sheets having high resiliency are used, the separation pawl must be manually retarded out of the sheet supplying path, thereby worsening the operability.